But that seems to depend on the way you develop the storylines. You can decide to have a relationship with emily right from the start, or after you have one with Mira. I did choose the relationship later. Contrary to the statement in the walkthorugh, Mira can now very well be persuaded to share with her mother. However, now you can fuck Emily as often as you want, Frisha won't see you doing it. So no chance for 3some. Maybe a bug?
If you want to see the threesome, you first have to have a relationship with emily, fuck her until Frisha watched, next built the realtionship ti Mira. In this order.